1. Field of the Invention
The present invention relates to an optical scanning device that scans a surface with a light flux, and an image forming apparatus that includes the optical scanning device.
2. Description of the Related Art
Demand has been increasing for image forming apparatuses (e.g., a laser printer or a digital copier) that can print a high-quality image at a high speed. To satisfy such demand, methods of scanning a scanning surface to be scanned with a light flux emitted from a multi-beam light source have been developed. Along with such development, various types of optical scanning optical systems have been developed.
For example, Japanese Patent No. 3445050 discloses a multi-beam optical scanning device including a lens with two surfaces each having a curvature ratio varying gradually from an optical axis in the sub-scanning direction, thereby preventing variation in f-number in the sub-scanning direction, which corresponds to image height of a light flux incident on a scanning surface.
Japanese Patent No. 3768734 discloses an optical scanning device that includes two scanning-imaging lenses having at least two surfaces whose sub-scanning-direction curvature ratio varies from the optical axis in the main-scanning direction. At least one of two surfaces has a sub-scanning-direction curvature ratio that varies in the main-scanning direction and that has a plurality of extrema.
Japanese Patent Application Laid-open No. 2005-338865 discloses an optical scanning device that includes a scanning lens with two surfaces each having a curvature ratio varying from the optical axis in the main-scanning direction. Specifically, the surface position on the optical axis in the sub-scanning direction is closer to a scanning surface than a surface position not on the optical axis. This structure prevents variation in f-number in the sub-scanning direction, which corresponds to an image height of a light flux incident on the scanning surface.
In the optical scanning devices disclosed in Japanese Patent No. 3445050, Japanese Patent No. 3768734, and Japanese Patent Application Laid-open No. 2005-338865, however, the increase in the number of light fluxes may increase variation in beam pitch in the sub-scanning direction. This may disadvantageously inhibit forming a high quality image at a high speed.